For years, Anders Behring Breivik participated in online debate forums on the dangers of Islam and immigration. Ahead of the massacre of over 90 people at a youth camp and the bombing of government buildings, the 32-year-old Norwegian was careful never to let anyone get wind of his intentions of mass murder.

He led a parallel life. When he was not plotting the mayhem or working on the bomb that he detonated on Friday here, he would be playing video games and blogging, listening to Euro pop and watching episodes of True Blood. Except on Sunday nights, when he usually dined with his mother, writes the New York Times.

Former classmates and colleagues describe Breivik as unremarkable and easy to forget.

It is not clear at what point he decided that violence was the solution to the ills he believed were tearing European civilization asunder.

"He didn't say anything you could remember," said Stig Fjellskaalnes, who knew Breivik when he was a member of Norway's conservative Progress Party in the early 2000s.

"He's one of the crowd, if you know what I mean. You forget him."

About a decade ago, Breivik started to change, though when a schoolboy he was fond of hip-hop and had a Muslim best friend. In his 20s he began to view the immigrants in Europe as enemies and those who sought to accommodate them as traitors, worthy only of execution.

"Around year 2000, I realized that the democratic struggle against the Islamization of Europe, European multiculturalism, was lost," he wrote in a 1,500-page manifesto that he published on the Web shortly before the attacks.

"I decided to explore alternative forms of opposition. Protesting is saying that you disagree. Resistance is saying you will put a stop to this. I decided I wanted to join the resistance movement."

With the manifesto, which he said took three years to complete, Breivik endeavored to find common cause with xenophobic right-wing groups around the world, particularly in the United States.

He quoted extensively from the anti-Islam writings of American bloggers, and cut and pasted a whole section of the manifesto written by Theodore J. Kaczynski, known as the Unabomber, into his own, replacing 'leftism' with 'multiculturalism' as the object of aspersion.

Breivik grew up in Skoyen, a middle-class district of western Oslo. His father, a civil servant, and mother, a nurse, divorced when he was one. Beyond that, his childhood seems to have been uneventful; Breivik said in his manifesto that it was happy.

He attended the elite high school where the country's current king, Harald V, and his son once studied. Former classmates remember him as quiet but intelligent, with a small rebellious streak: he was a prolific graffiti artist.

Toward the end of high school, he joined the youth wing of the Progress Party, drawn to its anti-immigrant platform and market capitalist bent.

He became estranged from his father, who moved to France. Then his sister, Elisabeth, on whom he seemed to rely in his father's absence, moved to the United States and married an American.

"Elisabeth was his only rock in life," said a former childhood friend, who would not give his name.

"When Elisabeth left, it really affected him. I think that's when Anders began to spiral."

It was a time when, according to his manifesto, his political views began to change. He began to perceive what he said was the hostility of Muslim youth.

He latched on to reports of attacks against ethnic Norwegian men and rapes of ethnic Norwegian women by immigrant gangs.

He wrote that the NATO bombing of Serbia in 1999 was a tipping point for him, describing the operation meant to halt a genocide as a betrayal of a fellow Christian people for the sake of Muslims.

He spent the next decade slowly working out his plan, though few people, it seems, had any inkling of it.

Four years ago, he joined the Norwegian Order of Freemasons. Ivar A. Skar, the group's leader, said Breivik has been 'excluded - the exclusion immediately effective'.

Breivik gathered six tons of ammonium nitrate fertilizer and turned aspirin powder into pure acetylsalicylic acid for his bomb. And all the while, he led an active life online, railing against Muslims and Marxists in debate forums.

He was also into fighting virtual demons, ogres and other fantastical creatures in online role-playing games. He was a regular in talk forums for players of World of Warcraft, using a busty female as his avatar and the handle, Conservatism.

The police say he rented a farm in eastern Norway not far from the capital and holed up there over the last several months to prepare his bomb.